The present invention relates to venting, and, more particularly, to a vent system for venting chambers.
Positive displacement meters are used to measure the volume of a liquid flowing through a line. For example, positive displacement meters are commonly used to measure the volume of oil transferred to a buyer. The meter is placed in the line that transports the liquid, and as the liquid flows through the meter it is metered.
A double case rotary positive displacement meter includes an inner mechanism with a rotor that is rotated by the liquid as it flows through the meter. Vanes, which are part of the rotor assembly, slide through slots to form measuring chambers that measure the volume of the liquid flowing through the meter. Typically, the vanes are constructed of aluminum, and the rotor is constructed of cast iron. The meter includes two housings, or cases. The inner mechanism housing encases the inner mechanism, and the outer housing encases the entire meter, including the inner mechanism.
During normal operation of the meter, the interior of the meter, including the entire interior of the rotor, is filled with the liquid. The liquid within the rotor, being essentially incompressible, enables the rotor walls to resist rapid pressure variations exerted on the rotor by the liquid outside the rotor. That is, when the rotor is completely filled with liquid, there is a balance of pressure between the exterior and interior of the rotor. However, gas pockets can occur within the rotor, typically when the meter has just been installed in a line or when the inner mechanism has been replaced. Gas pockets can also form from dissolved gas in the liquid during operation of the meter. When gas pockets are present within the rotor, the interior of the rotor is not incompressible, and rapid pressure variations from outside can cause the rotor to collapse, thus preventing the vanes from sliding within the slots in the rotor. This can cause major damage or destruction to the rotor and vanes. It is, therefore, desirable to vent the gas from the inner mechanism chamber and rotor.
Although gas pockets in the outer housing chamber will not damage the rotor, it is desirable to vent the outer housing chamber to improve the accuracy of the meter, and to prevent gas in the outer chamber from being passed downstream of the meter.
Because failure to vent gas from inside the rotor can cause failure of the meter, it is also desirable to provide some indication that gas is not present within the rotor, both prior to startup of the meter and during its operation.
One known system addresses this problem by providing a vent through the outer housing that vents the outer housing chamber through a first port, and a vent through the inner mechanism housing that is connected to a second port in the outer housing by a flexible hose. If used properly, this technique provides adequate venting during startup. However, the use of separate vents is awkward, and the lack of a gas detector makes its use during operation impractical. Also, care must be taken to choose a hose material that is compatible with the fluid being measured, and that can withstand the pressure within the outer housing chamber. Further, the hose that is connected to the outer housing cover must be connected with a coupling to the fitting that is connected to the port on the inner mechanism cover. The hose is relatively short and must be coupled before the cover is assembled. Therefore, the outer housing cover must be fitted in place and lifted as little as possible to enable coupling of the short hose to the fitting connected to the inner mechanism vent port. This operation is awkward and, in fact, dangerous, since an individual must reach under the heavy cover and into the outer housing chamber to make the connection, risking injury to the individual's arm if the cover should drop during the operation. It is desirable to eliminate the need for an individual to reach into the outer housing chamber to couple the hose to the inner mechanism vent port.